1. Accuracy of calculation procedures for offshore wind turbine support structures Pauline de Valk – 27 th of August 2013

2. Content  Introduction  Approach  Modeling  Results  Conclusions and recommendations 1

3. Content  Introduction  Approach  Modeling  Results  Conclusions and recommendations 1

4. Offshore wind energy shows potential to become one of the main energy suppliers  Demand for energy continues to increase  Offshore wind energy  More steady wind flow and average wind speed is higher than onshore Introduction Approach Modeling Results Conclusions and recommendations 2 Energy demand TWh

5. Offshore wind energy shows potential to become one of the main energy suppliers Energy demand TWh  Demand for energy continues to increase  Offshore wind energy  More steady wind flow and average wind speed is higher than onshore  Cost of energy (€/kWh) should be decreased  Structural optimization design 2 Introduction Approach Modeling Results Conclusions and recommendations

6. Optimize structural design of the support structure  Support structure one of the main cost items  In order to optimize one should have confidence in the outcome of calculation procedures 3 Introduction Approach Modeling Results Conclusions and recommendations

7. Thesis objective ‘‘Investigate the validity and conservatism 4 Introduction Approach Modeling Results Conclusions and recommendations

8. Thesis objective ‘‘Investigate the validity and conservatism of the current calculation procedures 4 Introduction Approach Modeling Results Conclusions and recommendations

9. Thesis objective ‘‘Investigate the validity and conservatism of the current calculation procedures for offshore wind turbine support structures 4 Introduction Approach Modeling Results Conclusions and recommendations

10. Thesis objective ‘‘Investigate the validity and conservatism of the current calculation procedures for offshore wind turbine support structures and propose improved procedures 4 Introduction Approach Modeling Results Conclusions and recommendations

11. Thesis objective ‘‘Investigate the validity and conservatism of the current calculation procedures for offshore wind turbine support structures and propose improved procedures based on these findings.” 4 Introduction Approach Modeling Results Conclusions and recommendations

12. Content  Introduction  Approach  Modeling  Results  Conclusions and recommendations 5

13. Offshore wind turbine support structure is custom engineered for every wind farm Introduction Approach Modeling Results Conclusions and recommendations 6 Foundation designer (FD)Turbine designer (TD)

14. Offshore wind turbine support structure is custom engineered for every wind farm Introduction Approach Modeling Results Conclusions and recommendations 6 Foundation designer (FD)Turbine designer (TD) (Adjust) design foundation 1

15. Offshore wind turbine support structure is custom engineered for every wind farm Introduction Approach Modeling Results Conclusions and recommendations 6 Foundation designer (FD)Turbine designer (TD) Integrate foundation model in aero-elastic model (Adjust) design foundation 12

16. Offshore wind turbine support structure is custom engineered for every wind farm Introduction Approach Modeling Results Conclusions and recommendations 6 Foundation designer (FD)Turbine designer (TD) Integrate foundation model in aero-elastic model Run aero-elastic simulation (and adjust tower design) (Adjust) design foundation 12 3

17. Offshore wind turbine support structure is custom engineered for every wind farm Introduction Approach Modeling Results Conclusions and recommendations 6 Foundation designer (FD)Turbine designer (TD) Integrate foundation model in aero-elastic model Run aero-elastic simulation (and adjust tower design) Extract interface loads/displacements between tower and foundation (Adjust) design foundation 12 4 3

18. Offshore wind turbine support structure is custom engineered for every wind farm Introduction Approach Modeling Results Conclusions and recommendations 6 Foundation designer (FD)Turbine designer (TD) Integrate foundation model in aero-elastic model Run aero-elastic simulation (and adjust tower design) Extract interface loads/displacements between tower and foundation (Adjust) design foundation Apply interface loads/displacements on detailed foundation model 5 12 3 4

19. Offshore wind turbine support structure is custom engineered for every wind farm Introduction Approach Modeling Results Conclusions and recommendations 6 Foundation designer (FD)Turbine designer (TD) Integrate foundation model in aero-elastic model Run aero-elastic simulation (and adjust tower design) Extract interface loads/displacements between tower and foundation (Adjust) design foundation Run simulation Apply interface loads/displacements on detailed foundation model 6 5 12 3 4

20. Offshore wind turbine support structure is custom engineered for every wind farm Introduction Approach Modeling Results Conclusions and recommendations 6 Foundation designer (FD)Turbine designer (TD) Integrate foundation model in aero-elastic model Run aero-elastic simulation (and adjust tower design) Extract interface loads/displacements between tower and foundation (Adjust) design foundation Run simulation Apply interface loads/displacements on detailed foundation model 6 5 12 3 4

21. Calculation post-processing analyses Introduction Approach Modeling Results Conclusions and recommendations 7 f wind f wave Dynamic analysis Force controlled g f wind f wave g

22. Calculation post-processing analyses Introduction Approach Modeling Results Conclusions and recommendations 7 f wind f wave Dynamic analysis Force controlled g f wind f wave Dynamic or Quasi-static f wave g

23. Calculation post-processing analyses Introduction Approach Modeling Results Conclusions and recommendations 7 f wind f wave Force controlled g f wind f wave ubub Dynamic or Quasi-static Displacement controlled Dynamic analysis f wave g ubub

24. Calculation post-processing analyses Introduction Approach Modeling Results Conclusions and recommendations 7 f wind f wave Force controlled g f wind f wave ubub Dynamic or Quasi-static Displacement controlled Dynamic or Quasi-static Dynamic analysis f wave g ubub

25. Calculation post-processing analyses Introduction Approach Modeling Results Conclusions and recommendations 7 f wind f wave Force controlled g f wind f wave ubub Dynamic or Quasi-static Displacement controlled Dynamic or Quasi-static Dynamic analysis f wave g ubub

26. Dynamic versus quasi-static analysis Introduction Approach Modeling Results Conclusions and recommendations  Dynamic analysis  Quasi-static analysis  Only accurate if structure is excited below first eigenfrequency 8

27. Dynamic versus quasi-static analysis Introduction Approach Modeling Results Conclusions and recommendations  Dynamic analysis  Quasi-static analysis  Only accurate if structure is excited below first eigenfrequency 8

28. Dynamic versus quasi-static analysis Introduction Approach Modeling Results Conclusions and recommendations  Dynamic analysis  Quasi-static analysis  Only accurate if structure is excited below first eigenfrequency 8 Frequency Amplitude ≈ 1 K __ ≈ 1 ω2Mω2M

29. Dynamic versus quasi-static analysis Introduction Approach Modeling Results Conclusions and recommendations  Dynamic analysis  Quasi-static analysis  Only accurate if structure is excited below first eigenfrequency 8 Frequency Amplitude ≈ 1 K __ ≈ 1 ω2Mω2M

30. Dynamic versus quasi-static analysis Introduction Approach Modeling Results Conclusions and recommendations  Dynamic analysis  Quasi-static analysis  Only accurate if structure is excited below first eigenfrequency 8 Frequency Amplitude ≈ 1 K __ ≈ 1 ω2Mω2M

31. Design cycle for offshore wind turbine support structure Introduction Approach Modeling Results Conclusions and recommendations 9 Foundation designer (FD)Turbine designer (TD) Integrate foundation model in aero-elastic model Run aero-elastic simulation (and adjust tower design) Extract interface loads/displacements between tower and foundation (Adjust) design foundation Run simulation Apply interface loads/displacements on detailed foundation model 6 5 12 3 4

32. Design cycle for offshore wind turbine support structure Introduction Approach Modeling Results Conclusions and recommendations 9 Foundation designer (FD)Turbine designer (TD) Integrate foundation model in aero-elastic model Run aero-elastic simulation (and adjust tower design) Extract interface loads/displacements between tower and foundation (Adjust) design foundation Run simulation Apply interface loads/displacements on detailed foundation model 6 5 12 3 4

33. Reduction of foundation to lower computation costs  Reduce large number of DoF into smaller set of generalized DoF  Size( ũ ) << size(u)  Lower computation costs  Approximation of exact solution  Reduction basis contains limited number of deformation shapes  Only accurate if  Spectral convergence  Spatial convergence Introduction Approach Modeling Results Conclusions and recommendations 10

34. Reduction of foundation to lower computation costs  Reduce large number of DoF into smaller set of generalized DoF  Size( ũ ) << size(u)  Lower computation costs  Approximation of exact solution  Reduction basis contains limited number of deformation shapes  Only accurate if  Spectral convergence  Spatial convergence Introduction Approach Modeling Results Conclusions and recommendations 10 =+ +

35. Reduction of foundation to lower computation costs  Reduce large number of DoF into smaller set of generalized DoF  Size( ũ ) << size(u)  Lower computation costs  Approximation of exact solution  Reduction basis contains limited number of deformation shapes  Only accurate if  Spectral convergence  Spatial convergence Introduction Approach Modeling Results Conclusions and recommendations 10 =+ +

36. Reduction methods Guyan reduction Introduction Approach Modeling Results Conclusions and recommendations 11 + …. + Static constraint modes

37. Reduction methods Craig-Bampton reduction Introduction Approach Modeling Results Conclusions and recommendations 12 Static constraint modes +++ Fixed interface vibration modes

38. Reduction methods Augmented Craig-Bampton reduction Introduction Approach Modeling Results Conclusions and recommendations 13 +++ Fixed interface vibration modes Modal Truncation vectors Static constraint modes

39. Impact on fatigue damage results  Offshore wind turbine exposed to cyclic loading  Fatigue is one of the main design drivers  Impact of error in the reponse on the accuracy of the fatigue damage results Introduction Approach Modeling Results Conclusions and recommendations 14

40. Content  Introduction  Approach  Modeling  Results  Conclusions and recommendations 15

41. Monopile versus Jacket Introduction Approach Modeling Results Conclusions and recommendations 16 EigenfrequencyOWT model [Hz] Foundation ω free [Hz] Foundation ω fixed [Hz] 1st0.306.7342.8 EigenfrequencyOWT model [Hz] Foundation ω free [Hz] Foundation ω fixed [Hz] 1st0.271.064.09

42. Wind, wave and operational loads  Wind loads  Random load, wide frequency spectrum  Excite frequencies up to 7 Hz Introduction Approach Modeling Results Conclusions and recommendations 17 ω Energy

43. Wind, wave and operational loads  Wind loads  Random load, wide frequency spectrum  Excite frequencies up to 7 Hz  Wave loads  Wave frequencies are generally lower  Excite frequencies up to 0.5 Hz Introduction Approach Modeling Results Conclusions and recommendations 17 ω ω Energy

44. Wind, wave and operational loads  Wind loads  Random load, wide frequency spectrum  Excite frequencies up to 7 Hz  Wave loads  Wave frequencies are generally lower  Excite frequencies up to 0.5 Hz  Operational loads  Rotation frequency of the rotor (1P)  Blade passing frequency (3P) Introduction Approach Modeling Results Conclusions and recommendations 17 ω Energy ω ω

45. Content  Introduction  Approach  Modeling  Results  Conclusions and recommendations 18

46. Introduction Approach Modeling Results Conclusions and recommendations 19 f wind f wave Dynamic analysis f wave Quasi-static post-processing analyses

47. Introduction Approach Modeling Results Conclusions and recommendations 19 f wind f wave g Quasi-static Force controlled Dynamic analysis g f wind f wave

48. Quasi-static post-processing analyses Introduction Approach Modeling Results Conclusions and recommendations 19 f wind f wave g Quasi-static f wave ubub Force controlled Quasi-static Dynamic analysis g f wind f wave ubub Displacement controlled f wave

49. Accuracy of quasi-static post-processing Introduction Approach Modeling Results Conclusions and recommendations 20 ω free Elastic energy in the foundation structure Energy [ J ] Excitation frequency [Hz] Energy [ J ]

50. Accuracy of quasi-static post-processing Introduction Approach Modeling Results Conclusions and recommendations 20 ω free ω fixed Elastic energy in the foundation structure Energy [ J ] Excitation frequency [Hz]

51. Expansion of reduced response  Response detailed foundation model obtained by expanding the reduced response of the foundation  Only accurate if model converges spectrally and spatially Introduction Approach Modeling Results Conclusions and recommendations 21 f wave ~ f wind f wave ~ Expansion Dynamic analysis

52. Spectral convergence Introduction Approach Modeling Results Conclusions and recommendations 22 Relative difference eigenfrequencies of reduced OWT model Frequency [Hz]

53. Spectral convergence Introduction Approach Modeling Results Conclusions and recommendations 22 Relative difference eigenfrequencies of reduced OWT model Frequency [Hz]

54. Introduction Approach Modeling Results Conclusions and recommendations 23 Relative energy difference of expanded response Expansion of reduced response Excitation frequency [Hz]

55. Introduction Approach Modeling Results Conclusions and recommendations 23 Residual correction Expansion of reduced response Excitation frequency [Hz] Relative energy difference of expanded response

56. Expansion of reduced response Introduction Approach Modeling Results Conclusions and recommendations 23 Residual correction Excitation frequency [Hz] Relative energy difference of expanded response

57. Post-processing analysis with reduced foundation in complete OWT model Introduction Approach Modeling Results Conclusions and recommendations 24 f wave f wind f wave ~~ Dynamic analysis

58. Post-processing analysis with reduced foundation in complete OWT model Introduction Approach Modeling Results Conclusions and recommendations 24 f wave g Force controlled Dynamic and Quasi-static g f wind ~ f wave f wind f wave ~~ Dynamic analysis

59. Post-processing analysis with reduced foundation in complete OWT model Introduction Approach Modeling Results Conclusions and recommendations 24 f wave g Force controlled f wave ubub Dynamic and Quasi-static Displacement controlled Dynamic and Quasi-static g f wind ubub ~ f wave ~ Dynamic analysis f wave f wind f wave ~~

60. Post-processing analysis with reduced foundation in complete OWT model Introduction Approach Modeling Results Conclusions and recommendations 24 f wave g ubub g f wind ubub ~ f wave ~ Force controlled Dynamic and Quasi-static Displacement controlled Dynamic and Quasi-static  Guyan reduction  Craig-Bampton reduction  Augmented Craig-Bampton reduction f wave f wind f wave ~~ Dynamic analysis

61. Post-processing analysis with reduced foundation in complete OWT model Introduction Approach Modeling Results Conclusions and recommendations 24  Guyan reduction  Craig-Bampton reduction  Augmented Craig-Bampton reduction f wave g ubub g f wind ubub ~ f wave ~ Force controlled Dynamic and Quasi-static Displacement controlled Dynamic and Quasi-static f wave f wind f wave ~~ Dynamic analysis

62. Post-processing analysis with Craig- Bampton reduced foundation in OWT model Introduction Approach Modeling Results Conclusions and recommendations 25 ω free ω fixed Relative energy difference with respect to exact solution Excitation frequency [Hz]

63. Post-processing analysis with Craig- Bampton reduced foundation in OWT model Introduction Approach Modeling Results Conclusions and recommendations 25 ω free ω fixed  Quasi-static post-processing inaccurate  ω free and ω fixed within excitation spectrum  Dynamic post-processing accurate  CB reduced model spectrally converged  Internal dynamics included Relative energy difference with respect to exact solution Excitation frequency [Hz]

64. Post-processing analysis with Craig- Bampton reduced foundation in OWT model Introduction Approach Modeling Results Conclusions and recommendations 25 ω free ω fixed  Quasi-static post-processing inaccurate  ω free and ω fixed within excitation spectrum  Dynamic post-processing accurate  CB reduced model converges spectrally  Internal dynamics included Relative energy difference with respect to exact solution Excitation frequency [Hz]

65. Fatigue damage - Jacket Introduction Approach Modeling Results Conclusions and recommendations 26 Relative damage difference with respect to exact damage ExpansionQuasi-static Force controlled Dynamic Force controlled Quasi-static Displacemen t controlled Dynamic Displacemen t controlled

66. Fatigue damage - Jacket Introduction Approach Modeling Results Conclusions and recommendations 26 Relative damage difference with respect to exact damage

67. Content  Introduction  Approach  Modeling  Results  Conclusions and recommendations 27

68. Conclusions  Following aspects tend to influence the accuracy of the calculation procedures:  The characteristics of the structure  First fixed and free interface eigenfrequency  Qs FC significantly underestimates fatigue damage for jacket  Use of a reduced foundation model in complete OWT model  Spectral and spatial convergence  Residual correction improves accuracy fatigue damage results  Post-processing method  Dynamic post-processing provides accurate fatigue damage results despite errors in interface loads/displacements Introduction Approach Modeling Results Conclusions and recommendations 28

69. Conclusions  Following aspects tend to influence the accuracy of the calculation procedures:  The characteristics of the structure  First fixed and free interface eigenfrequency  Qs FC significantly underestimates fatigue damage for jacket  Use of a reduced foundation model in complete OWT model  Spectral and spatial convergence  Residual correction improves accuracy fatigue damage results  Post-processing method  Dynamic post-processing provides accurate fatigue damage results despite errors in interface loads/displacements Introduction Approach Modeling Results Conclusions and recommendations 28

70. Conclusions  Following aspects tend to influence the accuracy of the calculation procedures:  The characteristics of the structure  First fixed and free interface eigenfrequency  Qs FC significantly underestimates fatigue damage for jacket  Use of a reduced foundation model in complete OWT model  Spectral and spatial convergence  Residual correction improves accuracy fatigue damage results  Post-processing method  Dynamic post-processing provides accurate fatigue damage results despite errors in interface loads/displacements Introduction Approach Modeling Results Conclusions and recommendations 28

71. Conclusions  Following aspects tend to influence the accuracy of the calculation procedures:  The characteristics of the structure  First fixed and free interface eigenfrequency  Qs FC significantly underestimates fatigue damage for jacket  Use of a reduced foundation model in complete OWT model  Spectral and spatial convergence  Residual correction improves accuracy fatigue damage results  Post-processing method  Dynamic post-processing provides accurate fatigue damage results despite use of reduced foundation Introduction Approach Modeling Results Conclusions and recommendations 28

72. Recommendations  Apply the different calculation procedures in BHawC with different load cases Introduction Approach Modeling Results Conclusions and recommendations 29

73. Recommendations  Apply the different calculation procedures in BHawC with different load cases  Set up clear guidelines for spatial convergence  Error estimation methods Introduction Approach Modeling Results Conclusions and recommendations 29

74. Recommendations  Apply the different calculation procedures in BHawC with different load cases  Set up clear guidelines for spatial convergence  Error estimation methods  Determine an efficient and accurate calculation procedure for more complex models Introduction Approach Modeling Results Conclusions and recommendations 29

75. Recommendations  Apply the different calculation procedures in BHawC with different load cases  Set up clear guidelines for spatial convergence  Error estimation methods  Determine an efficient and accurate calculation procedure for more complex models  Validate results with real OWTs and loads Introduction Approach Modeling Results Conclusions and recommendations 29

76. Thank you for your attention

78. Levelized Cost of Electricity Onshore LCoE Offshore LCoE Cost of electricity EEX Leipzig 100 10 1 4 198019902000201020202030 €/kWh

79. Fatigue damage computation Response StressesSN-curve Fatigue damage

80. Force versus displacement controlled  Force controlled approach  Displacement controlled approach f wave g ubub

81. Relative energy difference quasi-static analysis

82. Interface loads - Monopile

83. Interface loads - Jacket

84. Guyan reduced jacket in complete OWT model

85. Augmented Craig-Bampton reduction 1. External load represented by a spatial and temporal part 1. Quasi-static response and orthogonalize w.r.t. fixed interface vibration modes 2. Orthonormalize w.r.t. each other 1. Construct reduction basis

86. Augmented Craig-Bampton reduced jacket in complete OWT model

87. Facts wind energy Wind turbine Household  Power capacity  3 MW  Energy production  6 – 7,5 GWh per year  Serves ± 2000 households  Average household  2,2 persons  Energy usage  3500 kWh per year

88. Requirement for calculation procedures Detailed foundation in OWT model Reduced foundation in OWT model Expansion If spectrally and spatially converged Force controlled Dynamic If spectrally and spatially converged Quasi-static If ω free >> max( ω ext ) If spectrally and spatially converged If ω free >> max( ω ext ) Displacement controlled Dynamic If spectrally and spatially converged Quasi-static If ω free >> max( ω ext ) If spectrally and spatially converged If ω free >> max( ω ext ) ✔ ✗ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔